An ndata file may contain multiple data channels. For example,
the file may have 2 or more channels, each containing spike trains
recorded from a different neuron or a different electrode. Or, if the
file was created from a model that has hundreds or many thousands of
units in a population, there may be many channels with names
like mt_0_0, mt_0_1, etc.

One could analyze each channel by using many 'nda' commands, as follows:

chan_file - this parameter causes one nda command
to loop over a list of channel names. This is described in the section
on Common parameters.

With the 'nda_pop' command, pop simply refers to the
fact that there is a population of channels (more than one) to be
analyzed:

nda_pop all_chan plaid120.nda model.nd out.pl

This 'nda_pop' command will open the input .nd file and will find all
of the channel names in the first trial. It will then issue separate
"nda" commands (like those shown above) for each channel. A list of
options for running nda_pop is as follows:

all_chan - use all channels

nda_pop all_chan [nda_file] [nd_file] [out_file]

all_chan_prefix - use only channels whose name begins with
a specified prefix

nda_pop all_chan_prefix [nda_file] [nd_file] [out_file] [prefix]

all_chan_type - use only channels that have a certain
channel type, e.g., 0 for spike times, 1 for floating point data.

nda_pop all_chan_type [nda_file] [nd_file] [out_file] [chan_type]

all_chan_prefix_type - use only channels that have a
certain channel type and whose name begins with the specified prefix.

Special nda_pop parameters may be added to an .nda file to
improve the formatting of results for 'nda_pop', as follows:

nda_pop_flag 1 - The default value is 0 if this parameter
does not exist in the .nda script file. This can be used to inform
the nda analyzer that this analysis is part of a population
run. In this case, the nda analysis may (depending whether the
analysis code detects and acts on this flag) decide to use the channel
names to name any plots, or to include in any lines of statistical
outputs that are appended to files.

nda_pop_plotfile by_channel - The default value is "all"
if this paramter is not present in the .nda script file. By setting
this to by_channel, the analysis may (if it is implemnted) use
the nda [out_file] as a prefix and create a distinct filename for each
output plot by appending the channel name.

Analysis types that currently look for these flags include:

characterize_plaid

Future development.

This type of population analysis could be interfaced with the
look-up table analysis to customize the analysis for each channel.